Hydrothermal stability of different zeolites in supercritical water: Implication for synthesis of supported catalysts by supercritical water impregnation

Supercritical water (SCW) impregnation is an efficient and feasible method that has been used to prepare highly dispersed supported catalysts, but few studies have investigated the stability of support materials in supercritical water. Thus, our aim was to investigate the hydrothermal stability of zeolite supports (ZSM-5, TS-1, ZSM-35, HY, 13X, Beta, SAPO-11 and SAPO-34) as model compounds in supercritical water. Results showed that almost all of zeolites suffered from crystallinity change, structural properties degradation, obvious desilication and dealumination. The decrease of surface areas and the collapse of crystalline structures in HY, 13X, Beta, SAPO-11 and SAPO-34 were more serious compared to ZSM-5, ZSM-35 and TS-1. The micropore areas and acidity of all SCW-treated zeolites were reduced. 13X with lower Si/Al ratio had higher hydrothermal stability than HY due to the formation of extra-framework Al (EFAL). EFAl also generated strong Lewis acid sites determined by ammonia temperature-programmed desorption and Al-27 magic angle spinning nuclear magnetic resonance. Desilication and dealumination were simultaneous, and led to the increase of framework Si/Al ratio. ZSM zeolites (ZSM-5, ZSM-35 and TS-1) had higher hydrothermal stability than HY, 13X, Beta, SAPO-11 and SAPO-34 in SCW.